1. Field of the Invention
The present invention relates to a hydraulic brake apparatus for supplying a hydraulic brake pressure to each wheel brake cylinder mounted on each wheel of a vehicle, and more particularly to a apparatus which is provided with a hydraulic pressure boosting device.
2. Description of the Related Arts
As for hydraulic brake apparatuses, various types are known and already on the market, including the apparatus of a type provided with a hydraulic pressure boosting device. For instance, the U.S. Pat. No. 3,928,970 discloses a power-assisted master cylinder using a source of fluid under pressure, which includes a piston movable in a cylinder bore, a pressure space ahead of the piston having an outlet for connection to a brake circuit, a power chamber behind the piston having an inlet for connection to the source of the fluid under pressure, and a control valve assembly for controlling the supply of fluid from the pressure source to the power chamber. It was the object of that patent to provide the assembly in which a piston is pedal operated but which does not require a movable inlet or inlet seals which move under pressure. And, there is disclosed a master cylinder assembly which includes a first piston and a second piston, between which a first pressure space is defined and connected to a hydraulic pressure circuit. Ahead of the second piston, a second pressure space is defined and connected to a space behind the first piston. An inlet valve member is provided for introducing the pressure discharged from the pressure source into the second pressure space, and an exhaust valve is provided for communicating the second pressure space with a reservoir, and these valve members are adapted to be actuated in response to movement of the second piston.
Also, in order to increase a braking force when a rapid braking operation is made in case of emergency for example, Japanese Patent Laid-open Publication No. 9-24818 proposes a vehicle braking control apparatus, which includes a regulator for regulating power pressure discharged from a power source and supplying regulated pressure to wheel brake cylinders during a normal braking operation, and changeover device for communicating the pressure source directly with the wheel brake cylinders in case of an emergency braking operation. That publication discloses the regulator using a spool valve, and discloses device for varying an area for receiving the regulated pressure, so as to provide a desired characteristic of the regulated pressure to the master cylinder pressure. In practice, a resilient member such as rubber is placed ahead of the spool valve through an engaging member, to restrict the pressure applied to the spool valve, in accordance with increase of the area of the resilient member contacting with the engaging member. Also, Japanese Patent Laid-open Publication No. 9-24819 discloses an apparatus for increasing the braking force in case of the emergency braking operation.
According to the apparatuses of the above-described Japanese Patent Laid-open Publication Nos. 9-24818 and 9-24819, it is possible to increase the braking force in case of the emergency braking operation. However, the apparatuses provide only two kinds of characteristics of the master cylinder pressure to braking input force for the normal braking operation and the emergency braking operation. In other words, it is impossible to maintain a certain characteristic of a vehicle deceleration to the braking input force by varying the characteristic of the master cylinder pressure in response to variations of load to the vehicle and coefficient of friction of a brake pad, for example, to improve a braking property. In the U.S. Pat. No. 3,928,970 as described above, the apparatus having the inlet valve and exhaust valve is not adapted to increase the braking force in case of the emergency braking operation, different from those apparatuses as shown in Publication Nos. 9-24818 and 9-24819, so that it is difficult to improve the braking property of the apparatus.